Conventional nuclear reactors provide for solid uranium oxide fuel elements and solid moderator or control elements each typically clad in stainless steel, titanium or other durable material. The solid fuel elements and solid moderator or control elements are confined in a core according to a specific matrix so that their relative presence determines when the reaction is critical and the extent or output power of the reaction. A coolant is cause to flow through the reactor core and over the fuel and control elements serving thereby to cool these elements. Using the same primary coolant or by a heat exchanger using also a second or different coolant, this heat of the reaction is transferred to a steam generator whereby steam is generated and directed through turbines or the like to produce electricity.
The outlet coolant temperatures from the reactor are normally maintained in the region below 500.degree. C. and more typically 425.degree. C. The inlet temperatures of the primary coolant to the reactor is typically in the 250.degree. C. range. The effective atmospheric sink temperature is perhaps 25.degree. C. The maximum thermal differential between the outlet coolant and heat sink temperatures is thus of the order of 400.degree.-475.degree. C., to provide a maximum thermal efficiency of the reactor plant of the order of 30-35%.
Taking into account further the limited efficiencies of the various physical components in the energy conversion chain, the overall electrical output compared against the thermal energy potential provides an overall efficiency more typically in the range of 20-25%.
The Mialki Pat. No. 3,494,829 proposes a homogeneous thermonuclear fission reactor having a melt formed of a combination of the fuel and moderator or control elements. In the confinement of a closed reactor vessel circuit, gas is bubbled through the melt and is then passed over heat exchanger means where a secondary fluid is used to transfer the heat of the reaction to exterior apparatus to generate useful mechanical or electrical output. Also, the heated melt surface radiates a considerable percentage of the heat to the overlying heat exchanger structure. The temperatures of the melt are in the 1000.degree. to 2200.degree. C. range per the disclosure of the patent. However, the homogeneous fuel-moderator melt does not provide accurate control demanded of power reactors, but instead the specific dimensions and materials of the core melt determine selected operating characteristics at a range of outputs.
The Whittier Pat. No. 3,383,886 uses a solid fissionable fuel and a liquid control moderator settled over the fuel, and gas is bubbled through the liquid moderator and over the fuel as a means for controlling the reactor output. The patent notes that the gas bubbles in the moderator modify the density of the moderator to change the ratio in the core of the moderator atom to the fuel atom; and as the ratio changes from a low value to a high value, the reactivity likewise changes--first increasing, then peaking and finally decreasing. The patent notes that the reactor is undermoderated at the low output condition and overmoderated at the higher output condition. The fission heat is transferred by means of the circulating liquid moderator first spilling over at the top of the reactor core, falling by gravity through a heat exchanger, and then being pumped back into the reactor core for continued movement throughout the closed loop system. The drawbacks of this concept are the need for a pump to operate at the high temperatures of the liquid moderator, and the contra-needed reduction in the cooling capacity of the reduced gas bubbling at the high output end of the control.